Conventional thermoset networks of high cross link density, such as silsesquioxane resins, typically suffer from the drawback that when measures are taken to improve a mechanical property such as strength, fracture toughness, or modulus, one or more of the other properties suffers a detriment.
For example, increasing the toughness of various silicone compositions has been previously carried out by adding a silicone fluid to a silicone resin. U.S. Pat. No. 5,034,061 discloses a silicone resin/fluid polymer adapted to form a transparent, shatter-resistant coating. The composition includes a silicone resin copolymer consisting essentially of R.sub.3 SiO.sub.1/2 and SiO.sub.4/2 units with unsaturated olefinic functional R groups, a polydiorganosiloxane fluid with vinyl functionality, an organopolysiloxane crosslinker having hydrogen functionality and a catalyst. The composition is disclosed as being particularly adapted for use in coating incandescent glass lamps.
Canadian Patent 691,206 (1964) discloses the use of silica-filled silicone resin/fluid combinations for damping vibrations. The ability of the disclosed silicone resin/fluid compositions to dampen vibrations is illustrated through the measurement of the ratio of G', the elastic shear modulus, to G", the loss shear modulus. The magnitude of this ratio is indicated as being inversely proportional to the ability of the material to absorb vibration. The ratio of G'/G" of the subject materials is compared to that of compositions prepared without a resin constituent.
The above-described toughened silicone compositions are generally of the types having a fairly low modulus of elasticity. As used herein to describe silicone resins, the term "rigid" means that the resin material, in its unfilled condition, exhibits a certain "stiffness" characterized by having a Young's modulus of at least 0.69 GPa. As used herein, the term "unfilled" means that no reinforcing fillers, such as carbon or glass fibers or silica powders have been added to the resin.
Another method for increasing toughness of a silicone resin is by modifying the silicone resin with a rubber compound. U.S. Pat. No. 5,747,608 describes a rubber-modified resin and U.S. Pat. No. 5,830,950 describes a method of making the rubber-modified resin. The rubber modified-resin is prepared by reacting an uncured organosilicone resin and a silicone rubber. The resin and rubber can be reacted by addition reaction, condensation reaction, or free radical reaction. The resulting rubber-modified resin has a Young's modulus of at least 0.69 GPa in its unfilled condition. However, strength and toughness of the rubber-modified resin is generally inferior to tough organic polymers and still insufficient for some applications.
Rigid silsesquioxane resins have been employed in applications that take advantage of their heat- and fire-resistant properties. These properties make the silsesquioxane resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels. Thus, there exists a need for rigid silsesquioxane resins having increased flexural strength, flexural strain, fracture toughness K.sub.Ic, and fracture energy G.sub.Ic, without significant loss of modulus or degradation of thermal stability. In addition, rigid silsesquioxane resins have low dielectric constants and are useful as interlayer dielectric materials. Rigid silsesquioxane resins are also useful as abrasion resistant coatings. These applications require that the silsesquioxane resins exhibit high strength and toughness.
Therefore, it is an object of this invention to provide a curable composition that can be used to prepare a cured silsesquioxane resin having high strength and fracture toughness without loss of modulus. It is a further object of this invention to provide a method for preparing the cured silsesquioxane resin.